Railroads are under increasing pressure to reduce emissions and to increase fuel efficiency. One of several responses to these forces has been the development of hybrid locomotives. Donnelly has disclosed the use of a battery-dominant hybrid locomotive in U.S. Pat. No. 6,308,639 which is incorporated herein by reference. Hybrid locomotives can reduce emissions and fuel consumption in rail operations such as yard switching but they are less effective for medium haul freight or commuter trains.
In U.S. patent application Ser. No. 11/200,88 filed Aug. 19, 2005 entitled “Locomotive Power Train Architecture”, Donnelly et al. have further disclosed a general electrical architecture for locomotives based on plurality of power sources, fuel and drive train combinations. The power sources may be any combination of engines, fuel cells, energy storage and regenerative braking. This application is also incorporated herein by reference.
Multi-engine locomotives are not new and a number of configurations have been built over the years. Examples are the Baldwin locomotives built in the 1940s and, more recently, a dual engine locomotive built by the French Railway Company, VFLI. In U.S. patent application Ser. No. 11/201,267 filed Aug. 9, 2005 entitled “Multiple Engine Locomotive Configuration”, Donnelly et al. have disclosed a means of packaging engine modules on a multi-engine locomotive that optimizes the power density of the locomotive power plants while reducing emissions and fuel consumption.
In yard switching operations, for example, the engine or engines on a hybrid or multi-engine locomotive may be required to operate at low track speeds at either low or moderate output power for extended periods of time. Concurrently, the locomotive may also be required to provide its power at a reasonably high alternator output voltage level. This can be accomplished by operating the engine or engines at high rpm but at decreased fuel efficiency corresponding to a low output power level. At engine speeds below the normal operating range, output voltage from the alternator can be maintained by increasing the excitation to the alternator. However, at very low engine speeds, excitation cannot be increased without limit to maintain a high alternator output voltage and therefore the required output voltage cannot be maintained.
There thus remains a need for an alternator configuration that can operate at low engine speeds while continuing to provide a sufficiently high output voltage required by the traction motors and auxiliary power systems. This capability is preferred for more versatile hybrid and/or multi-engine control strategies that may require engines to operate over a wide range of rpms while providing electrical output at a sufficiently high voltage level to a common DC electrical bus.